1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of effectively performing an image fixing process.
2. Description of the Related Arts
Conventionally, a fixing station for use in an image forming apparatus employs a heat roller mechanism in which a fixing roller having a heat source and a pressure roller for applying a pressure to the fixing roller are provided so as to form a fixing nip through which a recording sheet is conveyed and is subjected to a fixing process. In such a heat roller mechanism, from its structure, the melted toner is inevitably separated from the fixing roller before it is sufficiently cooled off. Accordingly, an offset phenomenon is prone to be caused in which the toner is erroneously deposited on the surface of the fixing roller.
In recent years, a belt-type fixing mechanism capable of allowing the toner to sufficiently cool off has been looked at and various proposals associated with the belt-type fixing mechanism have been made.
In a Published Unexamined Japanese Patent Application No. 6-318001 (1994), one example of a belt fixing mechanism is disclosed, in which a seamless fixing belt is extended and is rotated between a heat roller internally having a heat source such as a halogen heater and a fixing roller, and a pressure roller is arranged to push the fixing belt against the fixing roller so as to form a fixing nip between the pressure roller and the fixing belt. In this mechanism, the toner is melted by a heat of the fixing belt heated by the heat roller, and the processes of fixing and cooling are performed at the fixing nip located downstream from the heat roller. The feature of this example is that, in order to prevent the offset phenomenon by reducing a temperature of the fixing nip, a recording sheet is made close to the fixing belt and is guided to the fixing nip so as to be sufficiently heated before reaching the fixing nip.
In general, the fixing belt of the belt-type fixing mechanism has a far smaller heat capacity than the fixing roller of the roller type fixing mechanism and, therefore, the fixing belt can rapidly be cooled off during the time when it is moved to pass through the fixing nip, resulting in an accurate prevention of the offset phenomenon.
On the other hand, this mechanism has a drawback that a sufficient fixing heat capacity cannot be obtained because of the small heat capacity of the fixing belt.
A Published Unexamined Japanese Patent Application No. 9-160405 (1997) discloses a technique which attempts to solve the above-mentioned problem. In this technique, a pre-nip is additionally formed at an entrance of an ordinary fixing nip formed by a pressure applied to the fixing roller by the pressure roller. The pre-nip is formed by winding the fixing belt around the pressure roller with a supporting roller arranged inside the fixing belt. Accordingly, the entire nip length is extended and, thereby, the recording sheet can contact the fixing belt for a longer time period so that a sufficient heat will be transferred onto the recording sheet.
When the velocities at which the recording sheet is conveyed in the image forming apparatus and in the fixing station are different, in particular, when the velocity at the fixing station side is relatively slower, the recording sheet may be slacked and tends to touch various portions of the fixing station. As a result of this touching, the surface of the toner image which is not fixed may be rubbed and the toner image may be damaged. This is often called an image rubbing phenomenon.
In the technique described in the above-mentioned Published Unexamined Japanese Patent Application No. 9-160405 (1997), in which the length of the fixing nip is made longer than usual, the recording sheet has a risk of touching the fixing belt before entering the fixing nip. This mechanism is explained below with reference to FIG. 1.
FIG. 1 shows a schematic representation of a belt-type fixing station which includes a fixing roller 100, a heat roller 104 internally having a halogen heater 102, and a fixing belt 106 extended between the fixing roller 100 and the heat roller 104. The fixing station further includes a pressure roller 108 for applying a pressure to the fixing roller 100 via the fixing belt 106 and a guide member 110 for guiding an incoming recording sheet 112 having an unfixed toner image on the surface thereof to a nip portion which is formed at an area where the fixing belt 106 and the pressure roller 108 are in contact under pressure. This nip portion is referred to as a fixing-process area N. The fixing-process area N is composed of a fixing-process area N1 and a fixing-process area N2. The area N1 is formed upstream from the area N2, which is the fixing-process area commonly used.
In the fixing station shown in FIG. 1, an entrance of the fixing-process area N is inevitably formed narrower because of the formation of the fixing-process area N1. If the recording sheet 112 is slacked, the surface of the unfixed toner image contacts the fixing belt 106.
In the type of fixing station illustrated in FIG. 2, when the recording sheet 112 is released at its trailing edge from transfer rollers (not shown) and becomes free during the time when the leading edge of the recording sheet 112 passes through the fixing-process area N, the recording sheet 112 is raised towards a tangent line A due to the stiffness of the recording sheet. This is referred to as a trailing edge rise phenomenon. With the trailing edge rise, the recording sheet 112 tends to contact the fixing belt 106 and, as a result, the image rubbing phenomenon is caused. Of course, a thicker recording sheet tends to cause more of a trailing edge rise than with an ordinary recording sheet.
Another example of the belt-type fixing station is described in a Published Unexamined Japanese Patent Application No. 9-90787 (1997), in which a seamless fixing belt is rotatably extended between a heat roller internally having a heat source and a fixing roller having an elastic layer, and a hard-structured pressure roller is arranged to push the fixing belt against the fixing roller so that a fixing nip is formed between the pressure roller and the fixing belt.
With this mechanism, the toner is melted by the heat of the fixing belt heated by the heat roller, and the processes of fixing and cooling are performed at the fixing nip located downstream from the heat roller.
Also, the elastic layer of the fixing roller is configured to have a heat-insulating function for protecting the fixing belt from losing unnecessary heat, as well as an elastically-deforming function for enlarging the fixing nip, having a thickness of at least 2 mm.
From the structure of the fixing station described in the above-mentioned Published Unexamined Japanese Patent Application No. 9-90787 (1997), it is understood that many of the belt-type fixing stations use a fixing roller having an elastic layer as well as a main driving roller for conveying a recording sheet. Also, it is understood that in many cases the position of the fixing roller is fixed in the fixing station because the driving force can easily be transmitted from an image forming apparatus to the fixing station.
However, when a fixing roller having a thick elastic layer is used as a main driving roller, a radius of the fixing roller measured from the center of the rotation axis to the fixing nip varies in an area between the leading and trailing edges due to deformation of the elastic layer and, therefore, it is difficult to reproduce the linear velocity of the fixing roller. Furthermore, the layer combining the elastic property and the heat-insulating property is prone to be worn and be deteriorated over time and, therefore, the linear velocity of the fixing roller becomes unstable. That is, the linear velocity can be known only when the fixing station actually operates and cannot be calculated. Therefore, the linear velocity of the fixing station cannot be specified during the design stage.
In addition, when the fixing roller located inside the fixing belt is used as a main driving roller, there is a risk of a slip occurring between the fixing roller and the fixing belt and, if the slip occurs, the linear velocity of the fixing roller is inhibited.
Another example of the belt-type fixing station is described in a Published Unexamined Japanese Patent Application No. 11-24486 (1999), in which a hard-structured pressure roller is applied with a force using a spring to push a fixing belt against a position-fixed fixing roller having an elastic layer so as to form a fixing nip between the fixing belt and the pressure roller. In this mechanism, the pressure roller is used also as a main driving roller. That is, such a roller as the pressure roller located outside the fixing belt is used as the main driving roller.
With this mechanism, the fixing nip is formed with deformation of the elastic layer of the fixing roller, which is not new, but the linear velocity of the fixing station may not be adversely affected by the slip occurring between the fixing roller and the fixing belt. Because the hard-structured pressure roller is used as a main driving roller for conveying the recording sheet, the linear velocity is highly stable. Therefore, the technique described in the Published Unexamined Japanese Patent Application No. 11-24486 (1999) can solve the drawbacks of the technique described in the Published Unexamined Japanese Patent Application No. 9-90787 (1997).
In the mechanism described in the Published Unexamined Japanese Patent Application No. 11-24486 (1999), the pressure and main-driving roller is movable in the direction of the thickness of the recording sheet orthogonal to the sheet transfer direction and a rotation force from a driving source is input to the pressure and main-driving roller from one side of the rotation axis of the pressure and main-driving roller. Therefore, the pressure varies in the direction of the axis of the pressure and main-driving roller depending upon the driving torque. As a result, the technique has numerous drawbacks such as generating wrinkles, an uneven glossy finish, a faulty fixing, an offset problem, etc.
FIG. 3 shows a schematic representation of a typical belt-type fixing station, such as the one described in the above-mentioned Published Unexamined Japanese Patent Application No. 11-24486. In this mechanism, a recording sheet 700 having an image is guided by a guide plate 702 such that the leading edge of the recording sheet 700 is guided to the surface of a hard-structured pressure roller 704 and is conveyed into a fixing nip area N.
However, in this mechanism in which the pressure roller 704 is applied with a force using a spring or the like to push a fixing belt 710 against the fixing roller 706 so as to form the fixing nip N between the pressure roller 704 and the fixing belt 710, there is a risk of displacement of the center of the pressure roller 704 from a position C0 to a position C1, as shown in FIG. 3, due to an elastic layer 708 of the fixing roller 706 which wears over time. In this case, an angle for the recording sheet 700 to approach is changed from θ0 to θ1 and therefore an entrance of the fixing nip N becomes narrow. As a result, the recording sheet 700 may be more prone to be jammed.
This jam problem caused by the change of the approach angle occurs also in the roller-type fixing station. In comparison with the structure of the belt-type fixing station shown in FIG. 3, the roller-type fixing station commonly has a structure in which the pressure roller having an elastic layer is arranged under the hard-structured fixing roller, in the case of fixing a mono-chrome image. In this structure, the elastic layer of the pressure roller is worn over timer and, therefore, the center of the pressure roller is moved towards the fixing roller. Therefore, when the leading edge of the recording sheet is guided by the pressure roller into the fixing nip, the above-mentioned jam problem may occur due to the change of the angle.
The fixing station using the fixing belt is described in various other publications including Published Unexamined Japanese Patent Application Nos. 8-137306 (1996), 4-273279 (1992), and 4-362984 (1992).